Abstract
We investigated the cell size reduction and size restoration of Eucampia zodiacus, an organism causing bleaching of aquacultured nori seaweed (Pyropia yezonensis), in laboratory culture observations. Mean cell size (apical length) of four E. zodiacus strains, which at the beginning of the experiment ranged between 59.2–73.4 µm, decreased to approximately 10 µm after 265–315 days with increasing number of cell divisions. The mean reduction rates in cell size ranged from 0.04 to 0.28 µm division-1 at 5 µm intervals of cell size. The rates were almost stable at cell sizes between 35 and 60 µm, and then declined with decreasing cell size. When the mean cell sizes reached <15 µm, they restored cell size. However, the restored-size cells ranged from 22.7–34.0 µm and were smaller than the maximum size (ca. 80 µm) observed in field samples from the Harima-Nada, eastern part of the Seto Inland Sea, Japan. Cell size restored to over 80 µm was observed only once, in the H31-C strain, when the mean cell size decreased to around 50 µm. The process of size restoration that was observed only once in the H31-C strain, was clearly different from the other size restoration mechanisms, suggesting that E. zodiacus has two different processes by which cell size is restored: sexual reproduction via auxospore formation and vegetative cell enlargement.
